Preparation of borohydrides



Sept. 23, 19,69 R Q RAlSOR ETAL 3,468,630

PREPARATION 0F BOROHYDRIDES Filed June 25, 1963 INVENTOR m/QMM A'lTORNEY United States Patent O 3,468,630 PREPARATIGN OF BOROHYDRIDESRalph C. Raisor and Raymond C. Rhees, Henderson, Nev., assignors, bymesne assignments, to American Potash & Chemical Corporation, acorporation of Delaware Filed June 25, 1963, Ser. No. 290,893

Int. Cl. C01b 6/08 U.S. Cl. 23-204 3 Clalms In the preparation ofpentaborane-9 by the pyrolysis of diborane, there are formed a number ofby-product intermediate boron hydrides such as B4H10, BHu, and others.Additionally, boron hydride polymers, typified as (BHx)n, are formed.The formation of such polymers is undesirable and must be minimized inorder to achieve a high yield or rate of conversion of diborane to thedesired pentaborane- 9 or other higher boron hydrides.

Heretofore, in an effort to increase the yield of desired higher boronhydrides anda corresponding decrease in the production of boronpolymers, it has been suggested that the diborane pyrolysis be carriedout in the presence of an inert liquid solvent such as cyclohexane,cyclopentane, n-heptane, and the like. Such a pyrolysis-withsolventprocess has proved to be quite satisfactory in the production ofdecaborane. Unfortunately, it does not provide a completely satisfactoryprocess for the production of pentaborane-9. The reason for this lies inthe nature of the solvent, the starting materials and the productproduced. Thus, in such a system, the diborane starting materials andhigher boron hydrides formed are soluble in the solvent. On the otherhand, evolved hydrogen and boron hydride polymers are insoluble in thesolvent. Thus, when practicing the process, pentaborane-9 dissolvesimmediately in the solvent and thereafter begins to react at once withthe yet-unreacted diborane initially dissolved in the solvent. Thereaction between pentaborane-9 and said diborane results in theformation of a higher boron hydride (decarborane) and polymer. lf thehydrogen evolved were soluble in the solvent, its presence therein wouldinhibit or substantially minimize the reaction between saidpentaborane-9 and the yet-unreacted diborane. So far, no completelysatisfactory means has been found for preparing pentabrane-9 in apyrolysis-with-,solvent system.

Surprisingly, however, it now has been discovered that pentaborane-9 andother desired higher boron hydrides may be prepared in high yields ifthe diborane pyrolysis 1s carried out in the vapor phase. To becompletely satisfactory, it is essential that the vapor phase reactionbe effected at a suitably controlled reaction temperature and for aprescribed reaction time in the presence of predetermined quantities ofhydrogen.

Moreover, it has been discovered that the presence of vapors ofcyclohexane, cyclopentane, n-heptane, and the like in thediborane-hydrogen feed gas favors the formation of low molecular Weightboron hydrides, viz, lower than decarborane, and increases the rate ofconversion of diborane to such boron hydrides.

Patented Sept. 23, 1969 'ice Broadly, this invention is carried out, inaccordance with one of its preferred modes of operation, vby introducingthe proper mixture of diborane, hydrogen and cyclohexane or the likeinto a reaction zone and holding it therein at a predeterminedtemperature for a prescribed reaction time suicient to form favorableproduct distribution and then quenching the reaction. Any boron hydridesformed in the reaction, before the formation of pentaborane-9 in thechain of pyrolysis reactions, may be separ-ated from the quenchedproduct and recycled to the reactor feed stream for further pyrolysis.It has been determined that quenching of the reaction may be achieved inplant scale units by intimately contacting the gases leaving the reactorwith a cold inert solvent.

For a more complete understanding of this invention, reference is madeto the following description taken in conjunction with the accompanyingdrawing which is -a schematic illustration of one form of apparatus inwhich the diborane pyrolysis may be carried out.

As shown on the drawing, diborane is fed initially through line 10 intoa reactor 12. The reactor may be formed of any material inert withrespect to diborane including, for example, stainless steel, glass, orthe like. Heat is supplied to the reactor in any convenient manner as,for example, by a surrounding jacket 14 lled with hot oil which entersthrough inlet 16 and exits through outlet 18.

A mixed gas stream comprising hydrogen, vaporous cyclohexane or the likeand vaporous low molecular boron hydrides also are introduced into thereactor through line 20. The resulting mixed gas streams enteringreactor 12 through lines 10 and 20 are heated and retained in thereactor at a temperature and for a period of time suiiicient to permitthe pyrolysis of the diborane to proceed to la point at which thedesired higher boron hydride, e.g., pentaborane-9, is formed.

The gases exiting from the reactor through line 22 enter an aspirator 24in which they are cooled by a stream of cold liquid cyclohexane enteringthrough line 26. The cold cyclohex-ane quenches the pyrolysis reactionand prevents the pentaborane-9 or other desired higher boron hydridefrom reacting with yet-unreacted diborane to form undesired higher boronhydrides, viz., decaborane or boron polymer.

It will be understood, of course, that the cold liquid cyclohexaneintroduced into the aspirator absorbs heat from the gases in the courseof quenching the same. Therefore, the liquid gas mixture leaving theaspirator is passed through a heat exchanger 28 to cool the liquidcyclohexane to make certain that the pentaborane-9 dissolved thereindoes not react further with dissolved diborane to form higher boronhydrides.

The liquid-gas mixture then is conveyed through line 30 to a phaseseparator '32 in which the liquid component of the mixture is allowed toseparate from the gases, The liquid component comprises a cyclohexanesolution of pentaborane-9 containing small quantities of diborane,tetraborane, pentabOrane-ll, decaborane and boron polymer.

The gas component comprises primarily hydrogen and diborane. Since thegasliquid mixture is in equilibrium in the phase separator, the gascomponent necessarily also will contain minor amounts of tetraborane,pentaborane-9 and pentaborane-ll.

Searated liquid is withdrawn from the phase separator 32 through line 34and is pumped by pump 36 through line 26 and back to the aspirator 24`where it quenches further quantities of gases undergoing pyrolysis inreactor 12. Approximately 10% by weight of the liquid is withdrawnthrough line 3-8 and sent to a product recovery station, not shown,wherein the pentaborane-9 is stripped from the cyclohexane. Thecyclohexane thus recovered is recycled to the phase separator throughline 40 together with cycloheXane make up, as needed.

The gases leave separator 32 through line 42 and are pumped by pump 44back through line 20 and into reactor 12. A portion of the gas is bledoff from the system through line 46 by means of pressure regulator 48.The diborane present in the gas is recovered by absorption in coldliquid cyclohexane in a recovery vessel, not shown, and reintroducedthrough line 40 into the phase separator.

This invention has been carried out at a number of differenttemperatures, for example, less than 200 C., 200 C., 225 C., and 240 C.,both with and without cyclohexane. The diborane feed rates and therecycle rates were varied to obtain desired changes in hydrogenconcentration and reactor retention time. In all cases, the pressure wasmaintained at one atmosphere. In those instances were cyclohexane wasnot present, the pyrolysis reaction was quenched by rapid cooling at -78C. Table I sets forth data of seven pyrolysis runs made at temperaturesless than 200 C.

Three runs were made at a temperature of 225 C. The data from these runsis set forth in Table III.

TABLE III Example Reactor temperature C.) 225 225 225 Condensertemperature 0.)- BQHB feed rate (m1./min.) System pressure (mm. Hg.)

Recycle ratio 37 9 37 3 17. 8

HzzBgH Ratio 3 04 4. 26 2 H2 in reactor feed (mole percent 75 3 81.0 83.8

Reactor retention time (sec.) 1 47 3. 66 10. 0

)32H6 conversion (percent) 70 5 72. 9 76. 7 Product distribution (wt.perce BHg:(B10Hi4 plus polymer) 5. 6 9. 0 5. 6

The data in Table III illustrates that higher pentaborane-9 yields areobtained at 225 C. than at lower temperatures. Using a hydrogenconcentration of about 82% TABLE 1 Example Reactor temperature C.) 110125 150 150 160 176 Condenser temperature C.) -78 -78 -78 -78 -78 -78BZH@ feed rate (m1./min.) 13. 3 26 4 28. 6 15 7 46. 7 19. 9 Systempressure (mm. Hg). 719 732 720 713 715 719 Recycle ratio 26. 6 29. 5 27.9 12.7 17. 3 41. 2 H2: B2B@ ratio- 0. 151 0 044 0.08 0. 23 0. 17 0. 17H2 in reactor feed (mole percent) 13. 2 4. 2 7. 4 18. 9 15. 7 37. 7Reactor retention time (sec.)- 3. 6 2. 8 2. 5 10. 1 2. 2 2. 2 BzHsconversion (percent) 1. 3 0. 21 12. 2 16. 6 11. 2 38. 2 06H12 feed rate(mg/min.) Product distribution (Wt. percent);

B4H1u 44. 2 20. 9 13. 0 18. 8 6. 5 10. 5

BH14 0 O 0 0 0 0. 1

Polymer; 0 0 0 0 0 0 Ratios:

B5Hg: (BiuHn plus polymer) The data in Table I illustrates that only onerun (Example 5) carried out at 150 C. and a retention time of 10.2seconds resulted in the production of any pentaborane-9.

Ten runs were made, in accordance with this invention, at a temperatureof 200 C. The results of those runs are set forth in Table II.

and retention times varying from 3.7 to 10.0 seconds,

pentaborane-9 yields of greater than 80 Weight percent are obtainable.

TABLE 1I Short retention times Optimum retention Times Example 8 9 10 1112 13 14 16 17 Reactor temperature (C.) 200 200 200 200 200 200 200 200200 Condenser temperature C.) -78 -78 -78 -78 -78 -78 -78 -78 -78 B- vHfeed rate (m1/min.) 47. 5 21. 9 13. 2 31. 9 25.5 39. 2 49. 0 15 1 10. 6System pressure (mm. Hg. 713 719 715 722 713 717 720 716 717 18 2 46 184 8 15.2 12.5 5 7 4 2 7.9 20.8 1 85 2 62 1. 50 1. 70 1 10 1 10 2.00 2.56

cent) O 72 0 59. 5 63 0 52 0 52. 6 67. 0 72 0 Reactor retention tim 1 81 6 3. 7 5. 8 1 8. 7 15.2 10 7 )32H5 conversion percent. 60 8 66 2 54. 143. 3 46 9 62. 3 57. 4 56 8 Product distribution (wt.

. 0.26 0. 39 0.43 0. 33 0.14 0. 18 1. 15 0. 09 0. 16 0. 12 B5Hg:(B4Hioplus B5H11) 0. 76 0. 45 0. 26 0. 69 1. 8 1. 6 1.6 2. 4 2. 5 3. 1B5HU:B10H14 plus polymer 7. 0 62. 0 42. 0 4. 8 7. 6 4. 3 2. 8 4. 8 2. 74. 7

The data in Table II illustrates that at a temperature of 200 C. theoptimum retention time for maximum pentaborane-9 production was 5 to 15seconds.

Eight runs were made, in accordance with this invention, at atemperature of 240 C. The results of those runs are set forth in TableIV.

TABLE 1V Example Reactor temperature C.) 240 240 240 240 240 240 240 240Condenser temperature -78 '-78 -78 -78 -78 -78 -78 -45BQHreedratetmL/mina- 12.4 11.2 12.2 12.2 13.3 10.9 9.5 13.2 Systempressure (mm.Hg.) 716 717 717 717 720 718 714 718 Recycle ratio 20.232.0 16.3 50.4 71.4 9.5 26.8 69.7 H2 BZHtratio 6.1 5.1 5.6 6.8 5.7 5.57.3 5.5 Hzinreactor feed (mole percent) 86.0 82.6 84.8 87.2 85.0 85.088.0 85.0 Reactor retention time sec.) 9.5 7.0 8.4 2.3 1.8 16.1 6.6 1.8B2B@ conversion (percent) 84.0 70.7 80.6 80.6 76.0 78.6 80.0 82.0Product distribution (wt. percent):

The data in Table 1V illustrates that increasing the may be made thereinwithout departing `from its true temperature of pyrolysis to 240 C. hadlittle effect on scope. increasing pentaborane-9 concentration in thereactor What is claimed is: products. The average pentaborane-9concentration .at 1. A process for converting diborane to a higher boron240 C. was 82.3 weight percent as compared to 81.8 25 hydrde whichcomprises heating a gaseous mixture of weight percent for optimumretention times at 225 C. diborane, hydrogen and vapor selected from thegroup While little benefit is gained at 240 C. as far as pentaconsistingof cyclohexane, cyclopentane and n-heptane to borane-9 production isconcerned, the ratios of desired a temperature within the range of fromabout 200 C. to product to other boron hydrides were more favorable 30about 240 C. for a period of from about 3.7 to about than they are whencarried out at 225 C. (Table 111). 15 seconds in a closed reaction zoneto form penta- The data further show that the most favorable conditionsborane-9 and immediately thereafter cooling the resulting for producingpentaborane-9 are a reactor retention time gaseous mixture to reduce therate of reaction of diborane of about nine seconds and a reactor feedgas containlng to a higher boron hydride. about 85% hydrogen. 5 2. Aprocess for converting diborane to pentaborane-9 A number of runs weremade having cyclohexane vapors 3 which comprises heating a vaporousmixture of diborane, present in the reactor feed gas. Comparable runswere hydrogen and an inert gas selected `from the group conmade Withoutcyclohexane. The results of these runs are sisting of cyclohexane,cyclopentane and n-heptane to set forth in Table V. Examples 29, 31, 33and 35 contained a temperature within the range of from about 200 C. t0cyclohexane. Examples 30, 32, 34 and 36 did not contain 40 about 240 C.for a period of from about 3.7 to about cyclohcxane. 15 seconds in areaction zone to form pentaborane-9, and

TABLE V Example Reactor temperature C.) 150 150 200 200 200 200 240 240Condenser temperature (C -78 -78 -78 -78 -78 -78 -78 -78 BZH. Feearate(m1./m1n.)- 17.1 15.7 17.2 19.5 22.2 10.6 15.0 9.5 System pressure (mm.Hg.) 724 713 719 719 717 717 717 714 Hgnreactor feed gas (mole percent)40 18.9 68 67 72 72 86 88 Reactor retention time (sec.) 10.2 10.1 8.38.2 9.2 10.7 6.4 6 6 B2H|eonversion (percent) 35 16.6 70 53 76 57 80 80Cyelohexane feed rate (mg./miu.) 79 0.0 52 0.0 46 0.0 62 0.0 Productdistribution (wt. percent):

84H1., 9.7 13.8 28.9 5.5 13.6 7.9 1.5 0.0 B5H9 3.5 0.0 50.9 61.4 56.265.0 88.2 87.0 B5H 87.1 80.2 12.6 20.2 29.1 13.4 0.6 0.0 13mm..- 0.0 0.06.7 12.5 0.6 12.4 7.3 7.0 (BH), 0.3 0.0 0.9 0.4 0.5 1.4 2.4 6.0 Ratios:

B11-1101135139 2.8 0.0 0.57 0.09 0.24 0.12 0.017 0.0 B5B.: (BiHm plusB5Hn) 0.036 0.0 1.2 2.4 1.3 3.1 42.0 0.0 135K.: (BmHn plus polymer) 11.76.7 4.8 51.0 47 9.1 6.7

The data set forth in Table V illustrates that the product immediatelythereafter quenching the resulting gaseous dSribUiOn iS effected by thePfeSenCS 0f lCliClOlleXlIl@ 111 reaction mixture to reduce the rate offormation of higher the feed gases for l'eaCOI' temperatures UP t0 at 1a5t boron hydrides by contacting said mixture with a quantity 240 C- At240 C th? only effec Was that .0f IOWel'lllg of a cold, liquid inertsolvent selected from the group the P01Ymef Concentration At 200 C,Condmlns for 65 consisting of cyclohexane, cyclopentane and n-heptane.the production of tetrabrane iver tlat of ht e 0d el 3, A process forconverting diborane to a higher boron boron hydrrdebs., At 130 d., cort1ltlons TOIt 62g? c hydride which comprises introducing diborane intoconuf exlltailare oeln; gae' pljoducts excep tact with a moving streamof a hot, vaporous inert gas c c o and h dro en maintalnln th r sultm a1x r the polymer, Whlch 1s reduced less than one-half 1ts normal 70 y gg e e g g seous m tu e amount.

While the invention has been described with respect to what areconsidered to be the preferred embodiments of this invention, it will beunderstood, of course, that cerat a temperature within the range of fromabout 200 C. to about 240 C. for a period of from about 3.7 to about 15seconds to convert the diborane to pentaborane-9, quenching theresulting stream of hot gases with a movtain changes, modifications,substitutions, and the like 75 ing stream of a cold, liquid inertsolvent, separating unhexane, eyclopentane and n-heptane.

References Cited UNITED STATES PATENTS 3,037,846 5/ 1962 Mann et al.23-204 3,078,530 2/ 1963 Reccardi et al. 23--204 3,167,392 1/ 1965Edwards et al. 23-204 3,169,829 2/ 1965 Johnston et al. 23-204 8, OTHERREFERENCES Owens: Journal of Applied Chemistry, vol. 10, pp. 483-493(1960).

Schechter et al.: Preparation of Pentaborane and the 5 Evalutation ofthe Hazards of Handling Diborane and Pentaborane, Report No.MSA-9973-FR, Navy Contact NOa(s) 9973, printed December 1950, declassiedMay 1954, pp. 27 and FIGURE 1.

10 OSCAR R. VERTIZ, Primary Examiner G. O. PETERS, Assistant Examiner

1. A PROCESS FOR CONVERTING DIBORANE TO A HIGHER BORON HYDRIDE WHICHCOMPRISES HEATING A GASEOUS MIXTURE OF DIBORANE, HYDROGEN AND VAPORSELECTED FROM THE GROUP CONSISTING OF CYCLOHEXANE, CYCLOPENTANE ANDN-HEPTANE TO A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 200*C. TOABOUT 240*C. FOR A PERIOD OF FROM ABOUT 3.7 TO ABOUT 15 SECONDS IN ACLOSED REACTION ZONE TO FORM PENTABORANE-9 AND IMMEDIATELY THEREAFTERCOOLING THE RESULTING GASEOUS MIXTURE TO REDUCE OF REACTION OF DIBORANETO A HIGHER BORON HYDRIDE.